This invention relates generally to devices for measuring torque and, more particularly, to a rotating shaft incorporating a torque-measuring device of the type described, for example, in publication FR-2692986.
The device of that publication comprises two magnetic-field generators that are secured to a first support and situated in the plane of a cross section of the shaft and two devices for detecting magnetic fields that are secured to a second support and immobilized in the plane of another cross section of the shaft. Such a device can be mounted directly on a rotating shaft and in theory does not require any local reduction in the rigidity of the shaft, bearing in mind the fact that the device is very sensitive to angular offset. In actual fact, the diameter of the shaft is nonetheless reduced slightly so that the torsional deformation of the shaft can be located axially. In any case, the shaft can withstand the whole of the operating torque without requiring additional means for limiting deformation due to torsion.
In some applications, the length available on the shaft for installing such a torque-measuring device is limited. This is particularly true of a motor vehicle steering box in which a rotating shaft needs to incorporate, along an installed length bounded on one side by the splines which mesh with the steering shaft and on the other sides by the rack pinion, the torque-measuring device as well as a ball bearing. As the accuracy of the torque measured by the device improves with the increase in separation of the points where the supports rest along the shaft, it is desirable for the supports to be placed as close as possible to the ends of the installed length, while at the same time saving space at the end of the installed length for mounting the rolling bearing.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
One approach to solving this problem might be, perhaps, to make one of the supports an extension of the inner race of the rolling bearing, the race being mounted tightly on the torsion shaft and held by an axial retention device of the circlip type. The outer race of the rolling bearing might be sweated into a rigid housing unable to rotate. In this type of assembly, besides the fact that the axial retention device takes up a certain amount of axial space, the axial and radial loadings applied to the torsion shaft would pass from the torsion shaft into the attachment housing via the rolling bearing. In some loading scenarios, immobilizing the torsion shaft with respect to the inner race of the rolling bearing might prove insufficient, this being all the more true if inconsistent assembly leads to a minimum amount of clamping, given the manufacturing tolerances. This might lead to relative movement, during operation, of the magnetic-field generators with respect to the torsion shaft and, therefore, to a relative movement during operation of the magnetic-field generators with respect to the members that detect magnetic fields. The initial setting would be altered and the torque-sensing device would therefore lose its setting and deliver an erroneous signal.
Another approach to solving the problem might be to mount the inner race of the rolling bearing around the point at which one of the supports rests on the shaft. However, this approach would require increasing the outside diameter of the rolling bearing and would, therefore, imply an overall size that would not be compatible with the rigid housing of the bearing and, more generally, with the space available in which to mount the bearing.